This invention relates to microstrip devices such as antennas, or filters, and to methods of tuning and manufacturing such devices.
Microstrip antennas and other devices need to be tuned because the dielectric of the material on which the printed element is supported is variable from batch to batch, and any changes in dielectric constant will affect the tuning. Tuning of the microstrip is carried out by either trimming matching stubs connected to the feeding cable by which energy is supplied to the device, or by modification of the printed pattern itself.
In one technique, the dielectric constant of the material is tested prior to manufacture and, according to this, an appropriate pattern for the printed element is selected. Because the element is generally formed by a photo-etching process, this requires a large number of different etching masks so that the appropriate mask can be selected to suit the value of the dielectric constant of the material. This procedure is complicated and expensive, and does not lend itself to large scale production.
An alternative technique involves the removal of areas of the printed pattern, after manufacture, until correct tuning is achieved. Again this technique is time-consuming and expensive; it is also difficult to control and requires skilled technicians to carry out.
Microstrip filters can be used to filter microwave energy supplied to a microstrip antenna so that the characteristics of the energy propagated by the antenna can be precisely controlled. Such filters take the form of a capacitor/inductance circuit and may form a part of the same board on which the antenna is formed. As with antennas themselves, the filter must be tuned accurately to produce efficient performance and this presents the same problems when the above-mentioned tuning techniques are used.